Long vertical shaft bioreactor systems suitable for the treatment of waste water by activated sludge processes are known and disclosed as for example, in U.S. Pat. No. 4,279,754 to Pollock.
A deep vertical shaft bioreactor system for the treatment of waste water, typically, comprises a bioreactor, a solid/liquid separator and intervening apparatus in communication with the bioreactor and separator. As fully described in aforesaid U.S. Pat. No. 4,279,754 such bioreactors essentially comprise a circulatory system which includes at least two substantially vertical side-by-side chambers in communication with each other at their upper and lower ends, with their upper ends being connected through a basin. The waste water for treatment is caused to circulate repeatedly through and between the downflow chamber (the downcomer) and the upflow chamber (the riser). Normally, the waste-containing liquor, referred to as mixed liquor, is driven through the circulating system by injection of an oxygen-containing gas, usually air, into one or both of the chambers. Typically, in a 500 feet deep reactor, air injection is at a depth of about 200 feet with the air at a pressure of 100 pounds per square inch. At start-up of the bioreactor, a mixture of air and influent waste water is injected into the riser in the nature of an air lift pump. However, once circulation of the mixed liquor begins, air injection can be also into the downcomer. The fluid in the downcomer having a higher density than the liquid-bubble mixture of the riser, thereby provides a sufficient lifting force to maintain circulation. Usually the basin is fitted with a baffle to force mixed liquor at the top of the riser to traverse a major part of the basin releasing spent gas before again descending the downcomer for further treatment.
Influent waste water is introduced at depth into the riser chamber through an upwardly directed outlet arm of an influent conduit. An oxygen-containing gas, usually air, is injected into the influent liquor in the outlet arm of the influent liquor conduit. In addition to oxygenating the waste liquor, the injected gas acts to create an air lift pump which draws the influent waste into the bioreactor riser. Effluent liquor is withdrawn from the riser through an effluent liquor conduit having its inlet located in the riser at a point below the outlet of the influent liquor conduit. During operation of the bioreactor the flow of influent liquor to and effluent from the bioreactor are controlled in response to changes in level of liquid in the connecting upper basin.
The injected oxygen-containing gas dissolves in the mixed liquor as the liquor descends in the downcomer to regions of greater hydrostatic pressure. This dissolved oxygen constitutes the principal reactant in the biochemical degradation of the waste. As the circulating mixed liquor ascends in the riser to regions of lower hydrostatic pressure the dissolved gas separates and form bubbles. When the liquid/bubble mixture from the riser enters the basin, gas disengagement occurs.
Reaction between waste, dissolved oxygen, nutrients and biomass substantially takes places during circulation through the downcomer, riser and basin bioreactor system. The products of the reaction are carbon dioxide, and additional biomass which in combination with unreacted solid material present in the influent waste water forms a sludge.
The term "Waste Water" as used herein is understood to include water carrying any type of biodegradable domestic and industrial waste materials, for example, normal domestic waste and the effluents produced by farms, food factories, refineries, pulp mill, breweries and other industries. By "mixed liquor" is meant the mixture of liquids and solids present in the bioreactor system.
Pressurized head tanks utilizing off-gas back pressures for bioreactor waste water flow control and off-gas treatment are disclosed, for example, in U.S. Pat. No. 4,272,379 to Pollock. In such designs, foam, air borne microbes, volatile organic compounds, and some biological solids are swept by off-gas from the head tank into an oxidiation tank through an injection pipe having one end submerged in liquid. The submergence of the pipe determines the back pressure on the shaft bioreactor and consequently the exit velocity of liquid in the deep extraction line. This liquid exit velocity is critical for subsequent successful flotation. This submergence also causes the generation of course bubbles and a partial fractionation of the foam. The off-gas serves to aerate and mix the liquid in the oxidation tank.
Aforesaid U.S. Pat. No. 4,272,379 describes a vertical shaft bioreactor comprising an enclosed head tank, a downcomer and riser operatively communicating with each other at their upper and lower extremities, communication at the upper extremities being through the head tank, means for directing influent waste to the riser, means for removing effluent waste from the riser, means for injecting an oxygen containing gas, normally air, into the waste within the riser and downcomer, gas conduit means in the head tank for venting gas therethrough into an adjacent collection vessel, the end of the gas-venting conduit in the collection vessel being immersed in a predetermined depth of waste liquid in the collection vessel, liquid conduit means in the head tank for venting liquid therethrough into the adjacent collection vessel, the end of the liquid venting conduit being immersed in the collection vessel liquid at a lower level than the end of the gas-venting conduit, the collection vessel having overflow conduit means for transferring waste liquid from the collection vessel to the bioreactor influent stream, the overflow conduit means being positioned to control the level of liquid in the collection vessel, thereby controlling the pressure exerted by the liquid upon the gas vented from the head tank, and thereby controlling the back pressure exerted by the gas upon the shaft.
The term "off-gas" means the gas from the bioreactor shaft effluent coming out of solution upon being recycled to the surface basin and lower pressure regimes of the bioreactor. Foam, air borne microbes, and volatile organic compounds (VOC's) present in the off-gas stream of a vertical shaft bioreactor system present environmental problems. In one prior art bioreactor system, the foam, along with some biological solids, is swept by the off-gas from the head tank into a foam, oxidation tank through an injection pipe having its end submerged in liquid. The submergence of the pipe determines the back pressure in the shaft and consequently the exit velocity of liquid in the deep extraction line. This submergence also causes the generation of course bubbles and a partial fractionation of the foam The off-gas serves to aerate and mix the liquid in the foam tank.
In a later prior art bioreactor system, an "air tuning" method was developed to regulate the flow of the deep extraction line in a step-wise fashion according to waste water feed rate. This was achieved by submerging off-gas feed lines at varying depths in the foam tank to give step-wise changes in back-pressure within the head tank.
Prior art teaches that when the aeration off-gases from a vertically oriented shaft bioreactor are contained in the head space above the liquid level in the head tank of the bioreactor, the pressure developed in the head space is equal to the pressure associated with the depth of submergence of the off-gas vent in the liquid in an adjacent open top vessel. When more than one off-gas vent is used, all vents of equal submergence operate at the same back pressure. Different head space pressures can be achieved by re-routing the off-gas into vents set at different submergence depths in the adjacent vessel. Re-routing the off-gas can be achieved by allowing the rise or fall of liquid level in the head space to "open" or "close off" vents of different submergence depth. This causes a step wise pressure change. However, only one head space pressure can exit at a time.
The present invention provides multiple gas confinement compartments in a single head tank. Each compartment can operate at a different pressure in response to back pressures created by equal submergence depth of off-gas vents in adjacent open-top vessels. Each of the vessels contains inert media on which biomass grow to cause different operating pressures in the respective biofilters. Changing head space pressures even during air scour and backwashing does not significantly change the level of liquid in a water column hydraulically connected to the head tank. Changing the liquid level in the hydraulically connected water column does not change the pressure in the head space. The head space pressures can change gradually responding to operational changes in the bio-filters.